The present invention relates generally to improvements in coding speech by hybrid companding (compressing and expanding) delta modulation and particularly relates to economical methods of implementing the codec or system with standard integrated circuits.
Adaptive delta modulation is a predictive coding system. It is basically the same as the adaptive differential pulse code modulation with a one-bit quantizer. Adaptive delta modulation systems presently known employ either syllabic or instantaneous companding schemes which are used separately. The purpose is to adapt the quantizer step size in response to variations of the input signal level. In a syllabically companding adaptive delta modulation system the one-bit quantizer step size is changed relatively slowly at a syllabic rate according to the variation of the input signal. On the other hand, in an instantaneously companding adaptive delta modulation system the quantizer step size is adapted to the input signal level instantaneously at every sampling time.
It is known that adaptive delta modulation yields significantly improved performance in signal-to-noise ratio and dynamic range over linear delta modulation. However, either one of the present adaptive delta modulation companding systems has shortcomings because a large amount of slope overload noise may occur whenever the input signal changes abruptly. Further, an instantaneous companding adaptive delta modulation such as constant factor delta modulation suffers a large degradation of the signal-to-noise ratio when bit errors occur. Also, continuously variable slope delta modulation which may be regarded as a representative method of the syllabically companding adaptive delta modulation has a relatively narrow dynamic range.
In this connection, reference is made to a paper by the Applicant and Hwang Soo Lee entitled "A Comparative Performance Study of Adaptive Delta Modulation Systems" which appears in Record of IEEE 1979, International Conference on Communications June, 1979, (Also appears in IEEE Transactions Comm. January, 1980).
This paper discusses adaptive delta modulation with syllabic companding where a one-bit quantizer samples the error signal in accordance with a desired clock or transmission rate. The quantizer step size is controlled by the syllabic compandor which includes a 3 or 4 bit shift register, a comparator and a low pass filter.
This syllabically companding system was compared with an instantaneous companding system. Here the quantizer step size is varied discretely at every sampling time. By means of computer simulation the paper compares the merits of these and other adaptive delta modulation systems.
Reference is also made to the patent to Hutchins U.S. Pat. No. 3,973,081, which relates to feedback residue compression for digital speech systems. While this patent has no direct bearing on the present invention, it is referred to for general background information.
It is accordingly an object of the present invention to overcome at least some of the shortcomings of the prior art as discussed hereinabove.
A further object of the present invention is to provide a hybrid companding delta modulation system for improving the performance of adaptive delta modulation systems.